This invention relates to trunked radio systems. In particular, this invention relates to more efficient use of communication resources used by trunked radio systems.
A trunked radio system is a two-way radio system that allocates a limited number of communication resources, such as radio frequencies, or time slots in a time division multiplexed communication system etc., on a time-shared basis amongst many users throughout a limited geographic area. The users time-share these communications resources on an as-needed basis by the users requesting a central controller for the privilege of temporarily using a resource to communicate with other subscribers to the system. If a communications resource is available for use when a user request one, the controller grants the request to the requesting user by broadcasting a message on a control channel throughout the coverage area, (hereafter control resource) which the users monitor. This message, an Outbound Signalling Word, (OSW) instructs the trunked radio system users of the identity of the channel assigned to the requesting user. (Other users may also be instructed to tune to the channel to permit communication with them to occur.) The requesting user may thereafter communicate with either a specific other user with whom communication was requested or other group related users, depending upon instructions broadcast from the resource controller.
Most trunked radio systems in operation today, are licensed to provide service throughout a geographic area on discrete frequencies, or frequence ranges. A trunked radio system license usually grants the licensee the right to the exclusive use of these communication resources in the particular coverage area. Of the communication resources used by a trunking system most trunked radio systems actually allocate pairs of resources or channels, where each allocated resource is comprised of a pair channels. One channel of the pair of channels comprising a communication resource is used for subscriber units to transmit on, (also known as a subscriber unit talk-in channel which the system controller listens to). The other channel of the pair of channels comprising a communication resource is known as a repeater talk out channel, (which the subscriber units listen to). These paired subscriber talk-in channels and repeater talk out channels are considered communication resources, although both a talk-in channel and a talk-out channel might also be considered as communication resources.
It is well known that trunked radio system communication resources are frequently unused for substantial periods of time. When a communication resource licensed for trunked radio use is not being used by the trunked radio system the communication resource is effectively being wasted since any other use of the resource is precluded. A more efficient use of available frequency spectrum might permit re-use of the licensed frequencies (or channels, time slots, or other communication resources) within the geographic area such that subscribers in different regions of the coverage area could use a communication resource simultaneously without interfering with each other. Ideally, both talk-in and talk-out channels would be available for re-use.
Since the breadth of the coverage area of the trunked radio system is primarily dependent upon the signal level radiated by both the resource controller and the subscriber units, the coverage area of a trunked radio system can be controlled by controlling the power levels of transmitters for the resource controllers and the subscriber units. FIG. 1 shown a schematic diagram of a trunked radio system (10) that provides radio communications to subscriber units (12) within a geographic region (14). This geographic region, (also referred to as the coverage area) (14) is the distance over which signals from a resource controller (16) (which is the administrator for the trunked radio system) have a useable signal strength, i.e. a signal level sufficiently high such that information therein may be recovered. Subscriber units beyond the coverage area (14) may be unable to detect signals from the resource controller (16). Similarly, subscriber units beyond the coverage area (14) might be too far from the resource controller (16) to provide the resource controller (16) with useable signals of their own. Reduced power levels of subscribers within the coverage area (14) and providing distributed resource controllers, also at reduced power levels, might permit a coverage area (14) as shown in FIG. 1 to be simultaneously partitioned into secondary, small-cell trunking systems providing communications throughout much smaller geographic areas within the larger area (14). A trunked radio system that permits more efficient use of limited radio spectrum, allowing the spectrum to be re-used, would be an improvement over the prior art.